Steering forces applied to a steering wheel by the operator are detected by a torque sensor that is mounted on a steering shaft coupled to the steering wheel. Based on the steering forces (steering torque) detected by the torque sensor, a controller of an electric power steering apparatus energizes an electric motor (hereinafter simply referred to as “motor”), which generates an assistive torque. The electric power steering apparatus transmits the generated assistive torque through a worm gear speed reducer mechanism, etc. to the steering shaft (steering system) for thereby reducing the steering forces that are applied to the steering wheel by the operator.
One known form of the torque sensor, as disclosed in Japanese Patent No. 3055752 (JP3055752B2) and Japanese Patent No. 2830992 (JP2830992B2), includes a torsion bar interconnecting an input shaft and an output shaft, and a core engaging the input shaft and the output shaft. When a torque is applied between the input and output shafts, the core is displaced, and the displacement of the core is electrically detected by a detecting coil. Another known torque sensor, as disclosed in Japanese Patent No. 3964414 (JP3964414B2) and Japanese Patent No. 4057552 (JP4057552B2), includes magnetostrictive films disposed on a steering shaft, and detecting coils for detecting a change in the magnetic characteristics of the magnetostrictive films. A torque applied to the steering shaft is electrically detected by the detecting coils.
Japanese Patent Publication No. 06-096389 (JPH06-096389B2) discloses a technology which, in the event of a failure of a torque sensor, cancels assisting of a steering force by the motor and makes a steering system manually operable when the vehicle speed is equal to or higher than a prescribed speed, and controls the motor depending on a steering angular velocity calculated from an output signal from a steering angle sensor when the vehicle speed is equal to or lower than the prescribed speed.